In the pulp and paper industry, pulp and paper mils have had in the recent past to focus on alternative treatments for reducing mill effluent, and consequently there has been significant interest and activity to move towards "mill effluent closure" or a minimum impact mill environmentally. Pulp and paper mills are actively pursuing technologies to reduce waste water effluent loadings of compounds resulting in BOD and COD as well as various organic and toxic compounds.
A prerequisite to moving towards "mill closure" is to adequately treat evaporator condensate before it can be reused in the Fiberline displacing hot water. The treated condensate should preferably be low in BOD, COD, toxicity and organic compounds so that these are not carried through to the Bleach Plant effluent.
The most common methods used to treat evaporator condensate are biological treatment and stream stripping.
At present, in the pulp and paper industry, membrane technology has been applied to mill effluent. It has not been proposed, however, to use reverse osmosis techniques for removal of undesired components from condensates. In this regard, U.S. Pat. Nos. 5,470,481 and 4,155,845 show methods and apparatuses for treatment of paper mill effluent. In greater detail, U.S. Pat. No. 5,470,481 shows a method and apparatus for recovering wash water from pulp and more particularly the method includes filtering the effluent to form a filtrate stream and an organic solids feed stream. The solids-containing stream is heated and pressurized to a super-critical state causing the stream to oxidize and form steam when cooled. This process results in a gaseous component, inorganic solids component and a liquid wash component. This system includes a reactor and separating means to recover the wash water. U.S. Pat. No. 4,155,845 relates to ultrafiltration of effluents, and particularly of soda-containing effluent. This method includes the addition of a flocculating chemical agent or agents to the pulp mill effluent followed by an ultrafiltration of the pre-treated effluent. The ultrafiltration steps are conducted under a differential pressure while maintaining a linear speed of the pre-treated effluent.
Accordingly, there exists a need to reduce the deficiencies of the prior art processes directed towards the treatment of the clean evaporator condensate and to provide a simple, economical and efficient treatment process for clean evaporator condensate, and particularly, combined 5th effect combined clean evaporator condensate. It has been found that combined evaporator condensate No. 5 when cooled to about 40.degree. C. may be passed through one and preferably two stages of reverse osmosis ("RO") membranes. This procedure removes a substantial portion of contaminants from the condensate, including contaminants resulting in BOD, COD, and toxins and organic compounds.